Figure 2

SPAMM tagging. (a) SPAMM pulse sequence. The tagging part consists of only two non-selective RF pulses (usually, 90° each), separated by the tagging gradient in the tagging direction, and followed by a large crusher gradient. The imaging part shows conventional Cartesian k-space acquisition (RO = readout, PE = phase encoding, SS = slice selection). This sequence creates parallel tag lines orthogonal to the x-axis. (b) Example of a SPAMM grid-tagged image showing left ventricle (LV) and right ventricle (RV). Note that this grid pattern needs the application an extra tagging stage (in the orthogonal direction) next to the first one before imaging takes place. Note also that the dark myocardium between the tag lines is not completely black due to longitudinal relaxation. (c) Illustration of spins evolution during different time points in the tagging stage, as follows: immediately before tagging application (time point 1), the magnetization (M) is at equilibrium state in the longitudinal direction. Immediately after the application of the first RF pulse (time point 2), the magnetization is tipped into the transverse direction by certain flip angle (45° RF pulses are assumed here for illustration). The tagging gradient then follows, which disperses the spins in the tagging direction (x-direction in this case), such that by the end of the gradient pulse (time point 3), the spins are modulated by incremental phase shifts along the x-axis (the figure shows all vectors emerging from the origin just for simplicity). The second tagging RF pulse tips the resulting modulated magnetization by another 45° into the transverse direction to result in spins modulated as shown at time point (4). A crusher gradient immediately follows to eliminate transverse magnetization components, leaving only the longitudinal parts, which show a sinusoidal pattern along the x-axis with values ranging from 0 to M.